OC单例模式的实现

#import <Foundation/Foundation.h>
@class SingleClass;

static SingleClass *instance = nil;

@interface SingleClass : NSObject <NSCopying>
+ (instancetype)shareInstance;
@end

@implementation SingleClass
//类方法，获取单例对象
+ (instancetype)shareInstance{
if (!instance) {
instance = [[self alloc] init];
}
return instance;
}
//类方法，调用对象的alloc方法时返回已存在的单例
+ (instancetype)allocWithZone:(struct _NSZone *)zone{
if (!instance) {
instance = [super allocWithZone:zone];
}
return instance;
}
//copy时，返回已存在的单例
- (id)copyWithZone:(NSZone *)zone{
return instance;
}
//copy时，返回已存在的单例
- (id)copy{
return instance;
}
@end

int main(int argc, const char * argv[]) {
@autoreleasepool {
SingleClass *s1 = [SingleClass shareInstance];
SingleClass *s2 = [SingleClass shareInstance];
SingleClass *s3 = [[SingleClass alloc] init];
SingleClass *s4 = [s1 copy];

NSLog(@"%p",s1);
NSLog(@"%p",s2);
NSLog(@"%p",s3);
NSLog(@"%p",s4);
}
return 0;
}

2015-03-22 22:24:21.201 SingleDemo[1349:61326] 0x1001066e0
2015-03-22 22:24:21.202 SingleDemo[1349:61326] 0x1001066e0
2015-03-22 22:24:21.202 SingleDemo[1349:61326] 0x1001066e0
2015-03-22 22:24:21.202 SingleDemo[1349:61326] 0x1001066e0